1. Field of the Invention
This invention relates to vehicle fuel tanks in general, and specifically to a tank vapor vent valve assembly.
2. Description of the Related Art
When closed, vehicle fuel tanks are subject to positive pressure fluctuations, due to the normal or diurnal generation of fuel vapor from the liquid fuel as it sits in the tank, and due to temperature increases. They are also subject to negative pressure fluctuations, due to temperature decreases, and due to liquid fuel being pumped out. The tank pressure must be kept within a normal range to assure proper operation. The normal positive pressure fluctuations caused by diurnal vapors, which used to be controlled simply by venting to atmosphere, are now controlled by routing to a vapor storage canister. Higher than normal positive pressure fluctuations, should they occur, are controlled by a pressure relief valve. Negative pressure fluctuations are generally controlled by a separate valve on the gas cap or tank, which inlets air to the tank below a certain threshold negative tank pressure. In the future, vehicles may also require tank vapor vent valve assemblies, often referred to as TVVA's, to also recover the fuel vapors displaced from the tank as it is filled, which are now generally just vented to atmosphere. Since this will require substantial additional structure on the vehicle, it would be desirable, if possible, to combine at least some of the tank pressure control functions with the TVVA structure, so as to minimize expense and complexity.